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CNSC Executive Summary, Regulatory Role of Probabilistic Safety Assessment

August 4, 2017

The technical report Regulatory Role of Probabilistic Safety Assessment includes information about:

  • the regulatory role of probabilistic safety assessments (PSA)
  • the complementary role of PSA to deterministic safety analysis (DSA)
  • what kind of major change would require an update to PSA models
  • whole-site PSA

What follows is an executive summary of the technical report.

What is a PSA?

A PSA is a comprehensive and structured analysis tool used to evaluate risk at a nuclear power plant (NPP). It examines the design and operation of an NPP to demonstrate the overall safety of the facility. The outcome of a PSA includes reports and computer models.

A PSA answers the following questions:

  • What could go wrong?
  • How likely is it?
  • What would the consequences be?

The CNSC requires two levels of PSAs for NPP licensees. A Level 1 PSA evaluates the sequences that could lead to severe reactor core damage, also known as reactor meltdown. At this level, the focus is on plant responses to different internal events, which could be initiated by human error or system malfunctions, and external hazards (such as earthquakes or floods).

Building on the results of the Level 1 PSA, a Level 2 PSA examines the containment response to the accident and assesses the likelihood and magnitude of potential radioactive releases to the environment. This assessment helps the CNSC and licensees better understand each NPP and identify potential safety improvements.

The CNSC PSA requirements were formally introduced to the Canadian regulatory framework in 2005.

PSA in the Regulatory Framework

The licensee is required to implement and maintain a PSA program. This is captured in the licence conditions handbook (LCH), which lays out the framework for complying with the associated nuclear power reactor operating licence.

In its regulatory role, the PSA supports the following regulatory activities:

  • regulatory oversight program to develop a risk-informed compliance verification process
  • changes to the licensing basis
  • life extension and long-term operation for nuclear facilities
  • risk-informed decision making
  • operational event evaluation and abnormal plant configurations

PSA forms part of one safety and control area (SCA) – safety analysis.

The CNSC Regulatory Framework defines 14 SCAs as technical topics used across all regulated facilities and activities to assess, evaluate, review, verify and report on regulatory requirements and performance.

Under the safety analysis SCA, safety goals are used as quantitative indicators of the overall safety of a NPP. The safety goals form part of the licensing basis, to identify design improvements to enhance safety and to ensure that the likelihood of accidents with serious radiological consequences is extremely low.

Safety goals are defined on a per reactor (unit) basis. This means that at a multi-unit reactor site, the safety goals are defined for each reactor. This practice is consistent with International Atomic Energy Agency (IAEA) recommendations. However, safety goals are not the sole basis for regulatory decisions.

The PSA results are compared against the established safety goals, among other indicators within the overall safety analysis SCA and other regulatory documents.

Like PSA, DSA is part of the safety analysis SCA. DSA tests the adequacy of plant safety based on design-basis accident scenarios. If the facility meets the requirements, the facility is judged "acceptable" in the sense that there is no "undue risk". But that risk is unquantified. PSA complements the DSA by quantifying undue risk (i.e., answering the question "how likely is it?") and determining the main contributors to risk. In this way, both PSA and DSA address all plant safety concerns.

CNSC staff review licensee-submitted PSA reports and computer models to ensure that they meet all regulatory requirements. There are two stages of a CNSC regulatory review of a PSA:

  • Stage 1 is focused on the overall PSA study, taking the completeness, consistency and coherence of the overall model into consideration and comparing it to the CNSC-accepted PSA methodology and the CNSC requirements in REGDOC-2.4.2, Probabilistic Safety Assessment for Nuclear Power Plants. The result enables CNSC staff to provide recommendations to the Commission on the PSA’s compliance with the regulatory document and accepted PSA methodology.
  • Stage 2 provides a detailed assessment of samples of the PSA model to assess the technical adequacy of the PSA. The results are used to support the safe operation of NPPs.

Major changes that would trigger PSA update

PSA models must be updated every five years, or more frequently if the facility undergoes major changes, in order to adequately represent the as-operated and as-built plant conditions. High-level guidance for identifying the major changes requiring an update of the PSA models is included in REGDOC-2.4.2 and supplemented by more detailed criteria and guidance in CSA N290.17, Probabilistic safety assessment for nuclear power plants.

The potential of whole-site PSA

The intent of a whole-site PSA is to evaluate the integrated risk of an NPP site, taking into account inter-unit and human interactions, and other radioactive sources.

PSA results could then be used to enable the emergency authorities to make more informed decisions for sheltering and/or evacuation and train operators.

Currently, PSA practitioners world-wide are facing technical challenges in reaching a consensus on an approach and methodology for a whole-site PSA. Regulatory requirements on whole-site PSA do not yet exist. Several international projects towards the development of whole-site PSA methodologies are still ongoing. The CNSC is taking the lead in two of the projects, one under the IAEA, and the other under the Organisation for Economic Co-operation and Development / Nuclear Energy Agency (OECD/NEA).

To obtain a copy of the full technical report, Regulatory Role of Probabilistic Safety Assessment, please contact the CNSC at or call 613-995-5894 or 1-800-668-5284 (in Canada).

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